1. Field of the Invention
The invention relates generally to electrical apparatus, and more particularly, to circuit breakers having current limiting capability.
2. Description of the Prior Art
Circuit breakers are widely used in industrial, commercial, and residential environments for protecting power distribution equipment against damage from overcurrent conditions. One measure of a circuit breaker is its current interrupting rating. This is defined as the highest value of available fault current which the circuit breaker can twice interrupt without suffering electrical or mechanical failure.
With the trend toward higher available fault currents, it is desirable to provide a circuit breaker with an increased current interrupting rating. This has traditionally been accomplished by using heavier components and by making the case of the circuit breaker larger and stronger so as to withstand the forces caused by high current operation. However, the space available for installation of circuit breakers is often limited, and it is desirable to increase the current interrupting rating while maintaining the same case size. This can be achieved by limiting the flow of current through the circuit breaker under fault conditions to a value less than the full available fault current. The amount of current flow through the breaker under these conditions is known as the let-through current.
If the speed of the breaker opening operation is increased, a higher arc voltage is produced in the first half cycle. This limits the peak let-through current and increases the current interrupting rating. One method of limiting the peak let-through current is to use magnetic forces generated by overcurrent conditions through the circuit breaker to rapidly force a movable contact arm away from the fixed contact. This method is used in the circuit breaker described in U.S. Pat. No. 3,815,059, issued June 4, 1974 to Leonard A. Spoelman and assigned to the assignee of the present invention. In this patent, a slotted magnetic device is disposed about the contact arm and generates sufficient magnetic flux upon overcurrent conditions to develop a force to draw the contact arm into the slot and separate the movable contact from the fixed contact. It is desirable to provide a more compact circuit breaker utilizing magnetic forces to provide improved current limiting action.
Arc extinction has been accomplished in previous circuit breakers by using a stack of spaced conductive plates to provide rapid arc extinction and prevent reignition of the arc after the first current zero. It is desirable to provide a circuit breaker including an arc extinguishing device which exhibits improved performance with no increase in size. It is also desirable to provide increased protection for the interior of the circuit breaker case by preventing the arc or hot gases generated by the arc from damaging components of the breaker operating mechanism.